Japanese yon and shi

[Ccaesar]

Hello everyone
I have spent the last two weeks trying to learn and memorise time, weeks, months, days, measurements, and numbers.
I noticed certain patterns, however, I would like some rules as to when you use shi/shichi instead of yo(n)/Nana and how come the "n" disappears at times, like in yotsu?

[Xenops]

Hello everyone
I have spent the last two weeks trying to learn and memorise time, weeks, months, days, measurements, and numbers.
I noticed certain patterns, however, I would like some rules as to when you use shi/shichi instead of yo(n)/Nana and how come the "n" disappears at times, like in yotsu?

Here is an explanation about Japanese numbers: Counting in Japanese

In short, it depends on whether the word is from Chinese or Japanese originally.

[devilyoudont]

Some observations about よ vs よん (exceptions may exist)

--It's possible to create borderline illegal stuff in the counter word because of ん... in which case よ tends to replace よん... your example is one of these... the counter is usually っつ, so we might expect it to be よんっつ except んっ is borderline impossible in Japanese.
--When avoiding し and using counters which use chinese numbers... it seems like よ tends to be preferred over よん (四時、十四日)

[Ccaesar]

Thank you very much for the explainations guys, it's much appreciated!